Switch-mode rectifiers (SMRs) have been used in telecommunications applications to provide a regulated 24 volt or 48 volt power source for communication switching systems and to charge back-up battery banks. Multiple SMRs along with the battery banks, a controller and a low voltage load disconnect (LVLD) device, such as a contactor or circuit breaker, are housed in a metal enclosure, usually referred to as a bay or a power plant. Multiple SMRs in one bay form a redundant power system. Because of the nature of telecommunications systems, the SMRs have been designed to be capable of "hot" pluggable. The power plant must maintain operation in the event that any one of the SMRs fail. When changing the failed SMR or during "hot" insertion, the power plant must continue to operate and to meet specified requirements. One such requirement is that the power plant output voltage measured at the point of plant regulation not overshoot or undershoot a specified percentage of the regulated power plant output voltage level.
One approach to provide the regulated output voltage during "hot" insertion is that SMRs are normally designed with a pre-charging feature where a long pre-charging metal pin or blade is connected in series with a pre-charging current limiting resistor and a diode. The pre-charging branch is then connected to the output filter capacitor banks of the SMR. The positive terminal of the SMR is intentionally made shorter than the negative terminal and the pre-charging pin as shown in FIG. 1. This configuration provides a short pre-charging time interval to allow the output capacitor bank of the SMR to be pre-charged. The pre-charging of the output capacitor bank minimizes a voltage dip or sag in the plant bus voltage when a SMR is "hot" inserted into the system. The voltage sag or dip may also trigger the SMR under-voltage shutdown.
To address SMR output noise, voice frequency and wide band, and EMI requirements, additional differential mode (Cdm) and common mode (Ccm) filter capacitors are normally required to be connected across the SMR output terminals and between the output terminals and the plant chassis as shown in FIG. 1. Since the SMR positive output terminal is usually connected to the plant chassis in telecommunication applications, the common mode filter capacitor between the negative output terminal and the chassis is "hot" inserted into the low voltage bus, since other SMRs are already connected into the low voltage bus and their positive terminals are connected to the chassis. Regardless of the pre-charging circuit of the hot plug-in SMR, the SMR may still cause output voltage to sag or undershoot, exceeding the specified system requirements. This undesirable condition is especially true for SMRs with their own output circuit breaker as a pre-charging device and without a pre-charging pin, since the output differential mode and common mode filter capacitors are usually located on the load side of the circuit breaker and the distance between the low voltage bus where the SMR is connected and the SMR output capacitor bank, or local voltage sensing point, is usually quite long. The circuit breaker is normally open during SMR "hot" insertion. The presence of the common mode and differential mode capacitors causes the voltage of the low voltage bus to sag. A voltage sag is experienced at the battery power plant output terminals regardless of battery bank on or off conditions.
A need has thus arisen for a battery power plant bus architecture to improve transient responses under SMR "hot" insertion conditions.